1. Field of the Invention
The present invention relates to the transmission of electrical carrier signals, and to the recording and reproduction of sound, and in particular to the reproduction, and the rerecording and playback, of previously recorded sound signals.
2. Description of the Prior Art
In playing back a sound recording, one hears not only a reproduction of the original signal, but also the undesirable extraneous noise components that are caused by imperfections in the original recording and in the storage medium, damage to the storage medium, and inherent defects inadvertently introduced by the metal parts used in the manufacture of the storage medium. Such noises commonly are designated by such terms as "surface noise", "crackle", "ticks", etc. These noises are generally of a first type of an audibly continuous nature distributed fairly uniformly throughout the audio frequency range and known as "white" noise and of a second type which are transient sounds of short duration and high amplitude relative to the signal. (As used herein, the terms "input signal" or "carrier signal" refer to the electronic signal corresponding to the music, message or other sound intended to be transmitted to the listener or receiver; the term "noise transient" refers to the undesirable impulses and similar disturbances which may be imposed on the carrier signal; and the term "total input signal" refers to the total signal being transmitted-- i.e. carrier signal plus noise).
Radio broadcast signals also may contain some additional noise components caused by disturbances in the transmission or reception, commonly called "static". Static, also, typically consists of pulses of short duration and high amplitude.
Previous processes in reducing unwanted noises in sound reproduction have relied on restricting the frequency range as far as is feasible for the purpose or application. Typically, the higher frequency components of the noise, usually referred to by such terms as "surface noises", "crackle", "ticks", etc. are suppressed throughout the playback, usually at some sacrifice to the fidelity of reproduction of the signal. Some sophistication in this process was introduced by Harmon H. Scott whose Dynamic Noise Suppressor used electronic means automatically to alter the frequency passband width in record reproduction in accordance with the loudness of the music. The Scott Suppressor takes advantage of the phenomena that the relative sensitivity of the ear to various frequencies varies with the loudness of the sound, and that louder passages of music and other audio signals have a correspondingly improved masking effect on the surface noise perceived from the recording; so that, in louder passages a wider frequency range is feasible in reproduction than is the case in the reproduction of the quieter passages for similar listening comfort and perceived tone quality. The operation of the Dynamic Noise Suppressor is, basically, a rather gradual one, especially in closing down the frequency range after loud passages, lest a too-evident change in background noise becomes audible. The Dynamic Noise Suppressor is incapable of suppressing transient noises save in a generalized manner by its relatively long-term modification of the pass band; it copes much more successfully with continuous "white" noise than with noises of a more pronouncedly transient nature. These same observations apply to such recent developments and extensions of the Scott principle as are embodied in U.S. Pat. Nos. 3,678,416 (Burwen) and 3,802,357 (Sachs). A different method of subjectively reducing noise is described in U.S. Pat. No. 3,275,326 (Welsh), in which two identical recordings playing simultaneously are auditioned.
Compressor-expander systems are well known in the art of sound recording and reproduction. These systems function by compressing the dynamic range in the recording process, and by expanding the dynamic range in the playback process to the extent that it had been compressed. The following U.S. Patents disclose various compressor-expander systems: U.S. Pat. Nos. 3,665,345 (Dolby), 3,729,693 (Dolby), 3,732,371 (Burwen), 3,813,559 (DeBoer), 3,815,039 (Fujisawa), 3,828,280 (Dolby), 3,829,715 (VanSluys), and 3,846,719 (Dolby). The systems are only effective for reducing noise in those sound recordings which are specifically pre-encoded for such systems.
Other assorted noise and signal reduction systems are disclosed in U.S. Pat. Nos. 2,736,711 (Hanson), 2,912,571 (Jacobson), 3,171,901 (Clemency), 3,394,235 (Schott) and 3,896,465 (Daizu).
There are also noise reduction systems known wherein noise transients are removed from transmitted signals by blanking operations wherein the entire signal is blocked from transmission for an interval intended to be co-extensive with a noise transient, and the charge on a capacitor replaces the signal during the blanking interval. Thus, U.S. Pat. No. 3,678,416 (Burwen) describes a circuit which utilizes a capacitor charged by low frequency energy from the input signal, and which substitutes the capacitor charge for the signal when a tick is detected; the duration of the blank being determined by the saturation delay of an amplifier in the control circuit; however, no provision exists for matching the instantaneous voltage on the capacitor with the instantaneous signal amplitude (signal plus noise) at either the initiation or termination of the blank. In U.S. Pat. No. 3,978,412 (Frerking), a switching device is described which "clamps" the audio signal for a predetermined blanking interval when a noise transient is detected, a blank is instituted and a capacitor engages to load down and replace the signal emanating from the noise detector. The latter switching device does not interrupt the signal transmission circuit.
We previously invented a system for reducing noise transients and filed U.S. patent application Ser. No. 686,107 on May 13, 1976 thereon. Our prior invention is concerned with suppressing only those noises in a carrier signal that are caused by discrete noise transients. Novel methods for suppressing these are incorporated in the system. One method takes advantage of the circumstance that imperfections in a recording, such as imperfections in the record material, defects in manufacture, damage, dirt, wear, mildew, etc. which may introduce transient noises into the reproduction of the recording, are not identical on opposite sides of the record groove, whereas the carrier signal is identical on opposite sides of the groove wall (when monophonic phonograph records are the medium wherein noise transients are to be reduced), save for such effects introduced in playback as tracing error, tracking error and "pinch effect" that turned out, in the application of our invention, to be of a second order nature. Our prior invention reduces transient noises in the reproduction of the recording by a continuous process of switching the reproduction to the one of the two groove walls having the quieter signal. An embodiment of our prior invention provides means by which the switching process can be between either of the groove side walls or the sum signal obtained by mixing the signals of the two side walls. Another method of suppressing transient noises in our prior invention comprises inserting a low pass or band pass filter in the transmission path for a predetermined length of time only long enough effectively to blank most noise transient.
There has recently been disclosed a system that suppresses noise transients by utilizing a time delay device and instituting a total blank for a time interval equal to or greater than the duration of the noise transient, starting at a zero crossing before the transient and concluding at a zero crossing sometime after a preestablished minimum interval. Various schemes for reducing impulse noises in audio signals, including impulse removal and splicing methods and the use of non-linear signal processors called "median filters", are discussed in Audio Engineering Society Preprint 1263, which is a report presented at the 57th Convention of the Audio Engineering Society, May 10-13, in 1977 in Los Angeles, Calif.